FIG. 1 shows the standard orientation directions when discussing wooden members. The longitudinal direction is along the fiber orientation, that is, in the direction of the trunk of the tree. The radial direction is across all the rings and therefore across the grain. The tangential direction is traveling with the ring. There are several established methods for determining the strength of such wooden members, specifically structural "2.times." material. A first method uses a bending device, referred to as a "Continuous Lumber Tester" or CLT. This device applies a fixed bending deflection to the wooden member as it passes through a series of rollers. The load presented by the member under a fixed deflection is an indication of its structural quality. A second method uses X-ray imaging to estimate the density of the material, and to image defects such as knots, which reduce the strength of the piece. A third method uses low frequency (audible) sound propagation characteristics along the length of the wooden member to determine an overall Modulus of Elasticity and damping.
The first method directly measures the bending Modulus of Elasticity (MOE) of the material and infers strength (tensile and compression), while the second method assumes that the strength is proportional to the density, which is correct to a first approximation. In fact, the Young's Modulus, or the Modulus of Elasticity, is related to the density times the square of the sound velocity. The third method employs an impact method to excite a longitudinal wave that reflects back and forth along the length of the wooden member; the time of the reflections is an indication of the sound speed, and therefore the Young's Modulus of the member. This latter approach has been previously disclosed in the prior art by Pellerin & Ross, for example.
Ultrasonic devices have also been used in the prior art to assess lumber strength. For example, the use of side mounted transducers firing in a cross pattern was disclosed in a paper by Rajeshwar et al. (B. Rajeshwary, D. Bender, D. Bray, K. McDonald, "An Ultrasonic Technique for Predicting Tensile Strength in Southern Pine Lumber", Trans. Am. Soc. Agri. Eng., 40(4):1153-1159, 1997)). FIG. 2 is an example of their transducer placement and ultrasound wave directions.
There are several drawbacks to each of the above-mentioned prior art methods. For example, the CLT uses a three point measurement system in which the load is placed at the center of the board, with supports for the board spaced 4 feet apart. Thus, for a typical 8 foot length board, only the center section is measured with any accuracy. As a result, these machines typically are followed by a manual visual inspection station, where a human operator looks for grade-limiting defects, especially in the end regions. There are obvious visual defects, such as edge knots or severe grain angles, which reduce the strength of the lumber in bending, tension, and compression. The CLT, since it measures the bending modulus, does not directly assess compressional strength along the wooden member. This is the reason that additional human grading, or "override" decision, is necessary. The CLT also has the drawback that if the wood is quite unsound it can break during the load testing. Since the wood travels through the machine at up to 1500 linear feet per minute, the broken wooden pieces can jam or otherwise maladjust the CLT, causing the entire measurement line to stop.
The X-ray Lumber Grader, or XLG, on the other hand, has the drawback that X-ray density is not completely indicative of strength. There are species and growing conditions in which the density of the wood is "normal", and yet the boards fail very quickly under load. In addition, many biological deterioration agents degrade the mechanical properties of wood yet do not change the density of wood. Thus, the data must be augmented. Several researchers have been experimenting with combining X-ray data with ultrasound measurements of sound speed to improve prediction accuracy. However, the resulting device is very complex. In addition, there is always a small risk involved in equipment that uses ionizing radiation.
The technique disclosed by Pellerin and Ross provides only an overall assessment of lumber quality, and does not properly account for the specific type and locations of defects that affect its ultimate utility as a strength member. Specifically, the location and orientation of knots, splits, and grain angle all affect the tensile and compression strength of the member, but the data from the reflected waveforms does not provide sufficient detail to account for these defects. Further, the technique disclosed by Pellerin and Ross is somewhat difficult to implement at high board feed rates, since it requires the boards to be relatively motionless while the ends are impacted and monitored. Further, there is often extraneous vibration energy in the wooden member due to machinery and handling equipment, which can cause errors in the estimation of reflected wave speeds.
The accurate grading of wooden members, for example, structural softwood lumber or hardwood pallet stock, requires that multiple characteristics of the wooden member be determined simultaneously. The overall stiffness and tensile strength of the wooden member must be measured in order to estimate the structural grade of the member. Fiber orientation, or grain angle, is also a primary determining factor in strength estimation. The location and size of defects, such as knots, splits, and decay or deterioration, also affect the structural properties of the material. Strength characteristics can be inferred from the sound propagation speed along the direction of the fibers. Therefore, the present invention is designed to provide a technique for use in identifying and measuring such characteristics while overcoming the above-mentioned limitations in the prior art so as to accurately assess the structural properties of wooden members for strength and/or quality grading purposes.